Oil producers utilize different means to produce steam for injection into the oil bearing formation. The steam that is injected into the geologic formation condenses by direct contact heat exchange, thus heating the oil and reducing its viscosity. The condensed steam and oil are collected in the producing well and pumped to the surface. This oil/water mixture, once the oil has been separated from it, is what is referred to as ‘produced water’ in the oil industry.
Since water can comprise up to 90% of every barrel of oil/water mixture removed from the formation, the recovery and reuse of the water is necessary to control the cost of the operation and to minimize the environmental impact of consuming raw fresh water and subsequently generating wastewater for disposal. Once the decision to recover water is made, then treatment of those produced waters is required to reduce the scaling and/or organic fouling tendency of the water. This treatment generally requires the removal of the hardness and other ions present in the stream, preferably to near zero. As is understood in the art, the ‘hardness’ causing ions are the combined calcium and magnesium salts in the water to be used in steam generation equipment and is typically expressed as parts per million (ppm) although other terms can be used. While silica is not considered as adding to the hardness value, its presence can also lead to scaling problems if present in other than minimal amounts.
The traditional method for generation of steam in enhanced oil recovery is to utilize a once-through steam generator (OTSG) in which steam is generated from a treated feedwater through tubes heated by gas or oil burners. The OTSG feedwater can have a total dissolved solids concentration as high as 12,000 ppm (as CaCO3 equivalent), but requires a hardness level that is near zero. This method produces a low quality or wet steam, which is 80% vapor and 20% liquid, at pressures ranging from 600 pounds per square inch gauge (psig) up to 2000 psig. This 80% quality steam is separated from the 20% water and then injected into the formation. Either a portion or all of the 20% blowdown is disposed as a wastewater. Another method that has been proposed to obtain the high quality steam requirement is using a water tube boiler instead of the OTSG to generate steam. The water tube boiler, however, requires an even greater amount of feedwater pretreatment than the OTSG to ensure problem free operation. For a comparison of the feedwater requirements for both OTSG and water tube boilers, refer to FIGS. 6 and 7. There are numerous ways to obtain the feedwater quality required for steam generation, several of which are outlined below for illustration.
The oil/water mixture coming out of the production well is sent to the primary oil-water separator where substantially all of the oil is separated from the produced water. This separator can be comprised of any known apparatus, but typically, it is comprised of one or more free-water-knock-outs (FWKO), which allow separation of the oil and water by gravity. The separated oil is further treated to remove the last of the water and then sent to storage.
The separated produced water is sent to a cone bottom tank where heavy solids, such as sand, are allowed to settle out and any remaining oil rises to the top for removal. If any substantial oil remains after this step, one or more induced gas flotation units are utilized to remove substantially all of the oil present in the produced water. Alternately, a de-oiling polymer can be used with a resultant waste oil/solids sludge, which needs further handling for disposal.
The de-oiled produced water stream is then further treated for reuse. Its constituents are variable but typically it is relatively high in total dissolved solids (TDS), total organic carbon (TOC), hardness, and silica. The water treatment plant schemes which have heretofore been utilized downstream of the de-oiling zone and upstream of the steam injection well, as well as the equipment which is necessary or desirable to obtain high quality steam at 600 psig, or greater, is the focus of the improvements explained and described in this disclosure.
Referring to FIG. 1, which outlines a typical prior art process used to obtain high quality steam for down hole injection, the raw produced water 6 is sent to a de-oiling process zone 8 and then to a warm lime softener 310. Chemicals 312 such as Ca(OH)2, Na2CO3, MgO, NaOH, and a coagulant are introduced into the lime softener depending on the reaction desired and a precipitate consisting of hardness and silica is generated. Following the lime softener, a media type filter 324 is utilized to remove the small suspended solids that were not caught up in the lime sludge. The partially softened produced water, still saturated in calcium (as CaCO3), is then further de-ionized in a weak acid cation (WAC) exchanger 18 which essentially removes all remaining divalent ions. The softened produced water is then sent to the once through steam generator 230, via a conduit that passes through pre-heaters (4 and 76), and 80% quality steam 236 is generated. A steam separator 240 removes the 20% water entrainment and produces high quality steam 100 for down-hole injection in the steam flooding process. The high temperature blowdown 96 from the steam separator is then sent to a series of flash tanks to provide progressively lower steam pressures for other uses. If zero liquid discharge is desired, then the flash steam 134 can be used in a steam driven multiple effect evaporator and crystallizer 140 to obtain a zero liquid discharge (ZLD) system.
This prior art method is known technology and is considered to be the industry standard. However, it carries with it several disadvantages. These are:
1. It has the highest chemical cost of any options
2. It has the highest cost for sludge and salt cake disposal requirements
3. The OTSG's are limited by the 80% conversion of water into steam
4. The OTSG has inherent design problems in terms of tube wetting, fouling, and scaling
5. In cold weather operations, the sludge from the lime softener becomes very hard to handle
6. In the event of an unscheduled maintenance shutdown, the sludge in the lime softener can quickly set up in a form similar to concrete and become very hard to remove from the system.
FIG. 2 depicts another current prior art process in which the lime soda softening, media filter, and polishing WAC are replaced by a mechanical vapor compressor evaporator (MVC) 244. The de-oiled produced water 14 may be treated with an acid such as hydrochloric (HCl) to lower the pH and destroy any non-hydroxide alkalinity present. Any non-condensable gasses (NCG) 58 present may be removed in deaerator 56. Caustic such as sodium hydroxide (NaOH) 62 may then be added to raise the pH to around 10 or higher. The MVC evaporator 244 produces a low TDS distillate stream 246 that is used to feed the OTSG 230 and the process of generating high pressure steam for down-hole injection is accomplished in the same manner as in FIG. 1. In this case, the blowdown 96 from the steam separator 240 is flashed to a steam driven crystallizer 252 which concentrates the brine blowdown 248 from the MVC evaporator 244 and thus provides a ZLD system. The low TDS vapor produced in crystallizer 162 is routed through conduit 166, where it is combined with the liquid portion 138 exiting the flash tank 130, and then to the OTSG feed storage tank 36.
While this process seems to provide a simple approach to providing high quality water to the OTSG, it has limited applicability in that the concentration of the hardness causing ions, such as calcium and magnesium, must be quite low in the raw produced water. If the hardness ions are not low, then the MVC is limited in the concentration factor obtainable, scale control chemicals are required, or it has to operate in the seeded-slurry mode to avoid calcium sulfate and silica scaling. In the seeded-slurry mode, calcium chloride (CaCl2) and/or sodium sulfate (Na2SO4) has to be added to the feed stream to ensure that a circulating magma of calcium sulfate (CaSO4) crystals, typically 3-10% suspended solids (SS), is maintained in the MVC evaporator 244. This circulating magma is used as precipitation sites for the incoming calcium ions and for the co-precipitation of silica. This seeded-slurry mode of operation is aptly taught in U.S. Pat. No. 4,618,429.
The disadvantages to this system are:
1. Power consumption is high due to MVC evaporator compressor
2. A very large electrical infrastructure is required to supply power to the MVC evaporator compressors
3. Suppliers of OTSG equipment are reluctant to design to greater than 80% quality steam even with high quality feed water
4. OTSG tube wetting problems
5. Applicability is limited to low calcium and low magnesium produced waters due to high pH requirements for silica solubility and even when low, acid cleanings are required to maintain evaporator efficiency by removing the CaCO3 scale that builds up.
6. The evaporator is subject to scaling from low solubility constituents in the evaporator feed like strontium, barium and complexes of metals that occur at high pH operation.
FIG. 3 is yet another prior art process that utilizes a MVC evaporator 244 to pre-treat the de-oiled produced water in the same manner as that shown in FIG. 2. In this case though, the high quality distillate 246 from the MVC 244 is cooled in heat exchanger 280 and sent via conduit 284 to a reverse osmosis unit (RO) 290 that removes the volatile TOC from it. The RO permeate 294 is then suitable for use by a high efficiency water tube boiler 110 that will produce high quality steam. The need for a steam separator system and blowdown condensate system is eliminated. Likewise, the inherent problems of an OTSG are thus eliminated and a greater conversion of water to steam is obtained. The blowdown from the boiler is directed to the MVC (262). The steam driven ZLD system of the preceding figures has to be eliminated in favor of a MVC driven system as the amount of blowdown from the water tube boiler is insufficient to support a steam driven evaporator. Due to compressor limitations, an MVC crystallizer 268 is also required for final concentration. On some produced waters, notably those with minimal non-volatile TOC, the RO system 290 is not required and the MVC distillate 246 is directed to the watertube boiler 110 without any further treatment. However, this variation has the potential of fouling and scaling the watertube boiler to a greater extent than when distillate post treatment is utilized.
The advantages of this system are the incorporation of the water tube boiler and a lower operating cost, due to lower fuel consumption, as compared to the MVC/OTSG FIG. 2 process.
The disadvantages of this system include:
1. Highest power consumption and highest electrical infrastructure requirements
2. High total cost compared to other options
3. Multiple types of MVC evaporators are required (pretreatment/blowdown and crystallizer) which complicates operation
4. TOC is all rejected to the pretreatment/blowdown and crystallizer MVCs which will likely cause foaming problems that will complicate operation and puts the MVC compressors at risk of damage                5. Pretreatment MVC evaporator distillate must be cooled prior to RO treatment and then reheated.        
In summary, the prior art process designs in current use for treating heavy oilfield produced waters for high quality steam generation to be utilized in down-hole steam flooding applications is not entirely satisfactory due to:
1. physical chemical treatment processes are usually extensive, require high maintenance and operator interface, and generate large sludge and regeneration streams that need to be dealt with in accordance with strict environmental regulations,
2. large quantities required of expensive treatment chemicals that, in cases, need special safety/handling procedures,
3. reliance on low efficiency OTSGs to generate high quality steam at a recovery rate of 80%, water to steam and the associated steam separator and blowdown condensate handling systems,
4. inherent OTSG problems with insufficient tube wetting, high heat transfer rates, and tube plugging,
5. high power consumption requirements and electrical infrastructure due to the use of vapor compressors,
6. treating the entire produced water stream to meet requirements for ASME grade water that can be utilized in a commercial water tube boiler.
As water is becoming increasingly expensive to treat, or in short supply, or both, it would be desirable to simplify the treatment necessary to generate high quality, high pressure steam and reduce the costs. Finally, it would be clearly desirable to meet such increasingly difficult water treatment objectives with better system availability and longer run times than is commonly achieved today.
It is believed that no one heretofore has thought it feasible to operate a water tube boiler on deionized water coupled to an evaporator system at high pH and at pressures high enough to provide steam that can be directly used for steam flooding projects. The conventional engineering approach has been to design systems such as those depicted in the prior art FIGS. 1-3 or to limit the final concentrations to levels that do not cause scaling problems.
Therefore, a heretofore unaddressed need exists in the heavy oil industry to address the aforementioned deficiencies and inadequacies. Accordingly, it would be advantageous to address the drawbacks to current practice, which would help both the environment and assist the production facility ownership and operations area in controlling costs.